ARM: at91/PMC: fix at91sam9n12 USB FS init

at91sam9n12 has Full-speed only USB. So we should add
it to the list in at91_pllb_usbfs_clock_init() function.
Moreover, at91sam9n12 has an unusual PMC in the sense that it
has a PLLB but also has a USB clock register.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Tested-by: Bo Shen <voice.shen@atmel.com>